1. Field of the Invention
The present invention relates to an apertured plate support mechanism used in a charged-particle beam instrument, such as an electron-beam lithography machine, and also to a charged-particle beam instrument equipped with the support mechanism.
2. Description of Related Art
A variable-area electron beam lithographic machine is known as a charged-particle beam instrument for writing a desired pattern on a substrate, such as a semiconductor wafer or mask substrate (reticle), by irradiating the substrate with a charged-particle beam.
In such an electron-beam lithography machine, an electron beam is emitted and accelerated from an electron beam source consisting of an electron gun. The beam is passed through an aperture formed in an apertured plate so that the cross section of the beam is shaped. The beam shaped in this way is made to hit a specified position to be written on the substrate for a desired time under control of an illumination system having an objective lens and a deflector. Lithographic delineation on the substrate using the electron beam is carried out by repeating the irradiation by the shaped beam with different specified positions.
Usually, there are fitted two apertured plates, each having an aperture for shaping the cross section of an electron beam. A deflector is disposed between the two apertured plates. The shaped beam passed through the aperture in the first plate is made to hit the second plate having the aperture that is offset from the aperture in the first plate. In consequence, the cross section of the beam passed through the aperture in the second stage is shaped into a desired form.
FIG. 1 schematically shows a mechanism for supporting such an apertured plate. In this figure, the support mechanism is generally indicated by numeral 51. This mechanism 51 has a plate holder 51a for holding the apertured plate and a holder support 51b for supporting the holder 51a. 
The plate holder 51a has a holder body 59 and a cap portion 60. The apertured plate is held to the plate holder 51a while sandwiched between the holder body 59 and cap portion 60. A protruding portion 61 for engagement is fixed to the holder body 59. When the apertured plate held to the plate holder 51a is replaced, a holder pullout (not shown) is brought into engagement with the protruding portion 61 of the plate holder 51a to move the plate holder 51a. 
The holder support 51b has a rectangular upper plate 52, a U-shaped pedestal 58, and a connector portion 57 for connecting the upper plate 52 and pedestal 58. The plate holder 51a is accommodated in the space between the upper plate 52 and pedestal 58 and supported on the upper surface (supporting surface) 58a of the pedestal 58. At this time, the lower supported surface 59c of the peripheral portion of the holder body 59 of the plate holder 51a is in contact with the upper surface 58a of the pedestal 58.
A recess 59b is formed in the side surface of the holder body 59 of the plate holder 51a . A positioning pin 56 mounted between the upper plate 52 and pedestal 58 is fitted in the recess 59b, thus placing the plate holder 51a in position.
A U-shaped leaf spring 54 is mounted to the lower surface 52b of the upper plate 52 of the holder support 51b via a holding portion 53. A roller 55 is mounted at the front end 54a of the leaf spring 54. The bottom of the roller 55 is in contact with the upper surface 59d of the holder body 59 in the plate holder 51a. Resilient force of the leaf spring 54 holds the plate holder 51a against the pedestal 58, the spring 54 being mounted to the upper plate 52.
An aperture 52a is formed in the upper plate 52 of the holder support 51b. Also, apertures 59a and 60a are formed in the holder body 59 and cap portion 60, respectively, in the plate holder 51a. The apertures 52a, 59a, and 60a are so positioned that their centers are coaxial. An electron beam 100 emitted from an electron beam source (not shown) passes through the apertures 52a, 59a, and 60a. 
The apertured plate is held between the holder body 59 and cap portion 60 in the plate holder 51a. The center of the aperture in the plate is made coaxial with the centers of the apertures 52a, 59a, and 60a. When the beam 100 passes through the apertures 59a and 60a formed in the holder body 59 of the plate holder 51a and cap portion 60, the cross-sectional shape of the beam 100 is shaped.
FIG. 2 is a plan view showing main portions of the support mechanism 51 for supporting an apertured plate 62 which is provided with an aperture 62a. The apertured plate 62 is disposed under the holder body 59. The apertured plate 62 is held between the holder body 59 and cap portion 60 and fixed, to the holder body 59. As described previously, the center of the aperture 62a formed in the apertured plate 62 and the center of the aperture 59a formed in the holder body 59 are coaxial.
The position of the plate holder 51a within a plane is determined by contact of two contact surfaces 58b, 58c of the pedestal 58 with the side surface 60b of the cap portion 60 forming the plate holder 51a. 
In another known structure, such an apertured plate is supported by support pins. Thus, the apertured plate is supported to an apertured plate support mechanism (see Japanese Patent Laid-Open No. 10-284391, for example).
In the plate support mechanism 51 constructed as shown in FIGS. 1 and 2, the electron beam 100 may hit the plate holder 51a. This thermally expands the cap portion 60 forming the plate holder 51a. 
When the cap portion 60 forming the plate holder 51a thermally expands in this way, the apertured plate 62 moves away from the contact surfaces 58b and 58c of the pedestal 58. As a result, the aperture 62a formed in the apertured plate 62 moves away from the contact surfaces 58b and 58c. 
If the aperture 62a formed in the apertured plate 62 moves unnecessarily in this way, shaping of the cross section of the electron beam 100 passing through the aperture 62a is affected. Furthermore, the amount of current supplied from the electron beam 100 to the substrate and the writing position are varied.
In the technique described in the above-cited Japanese Patent Laid-Open No. 10-284391, four support pins are disposed rotationally symmetrically with respect to the center of the apertured plate. The support pins act to support the apertured plate, each with a point contact with the plate. It is said that if the apertured plate holder thermally expands, the resultant displacement of the plate holder can be absorbed by slippage of the point contact portions.
It is considered that in this structure, too, movement of the apertured plate and the aperture formed in it due to thermal expansion of the plate holder can be prevented to some extent. Since displacement of the plate holder is absorbed by slippage of the point contact portions, it is difficult to control the slippage reliably. It is difficult to certainly prevent movement of the apertured plate and aperture. In addition, it is difficult to restrict rotation of the apertured plate within a plane.